The present application claims priority based on United Kingdom application Ser. No. 9725345.4 filed Nov. 28, 1997.
1. Field of the Invention
This invention relates to horn loudspeakers and loudspeaker systems.
2. Prior Art
Horn loudspeakers are well known and typically comprise a horn, which may have, for example, a conical, exponential or hyperbolic taper, with a throat and mouth, and an electro-acoustic driver mounted at or adjacent the throat of the horn and directed generally along the horn.
The horn loading of the driver offers significant increases in overall electro-acoustic efficiency and can control the radiating pattern of the driver. Unfortunately, the pattern control achieved by horn loading a loudspeaker is imperfect and is frequency dependent, despite the claims of so-called constant directivity horns.
The directivity of a well designed horn is reasonably constant down to a lower limiting frequency. Below this frequency, the directivity decreases significantly and the horn loses its directional control. The frequency at which directivity control is lost is inversely proportional to the size of the horn mouth, making it difficult to produce small horns with good control of low frequency directivity. See for example Henricksen and Ureda xe2x80x9cThe Manta-Ray Hornsxe2x80x9d, Journal of the Audio Engineering Society, 1978, who suggest an expression for the break frequency below which pattern control is lost of form:       f          break      =              k                  θ          ⁢                      xe2x80x83                    ⁢          X                      ⁢      xe2x80x83  
where
X horn mouth size (m)
xcex8 Coverage angle (degrees)
K constant: 25400 (degree metres/Hz)
The horn controls the acoustic radiation impedance seen by the driver, and the horn profile couples the radiation load at the throat to the acoustics of waves in free air after the mouth. The profile of the horn causes a changing acoustic impedance for waves propagating from the driver, down the horn, and out into the listening space. This changing impedance influences the polar response of the horn.
In accordance with the present invention, there is provided a horn loudspeaker, comprising: a horn having a throat and a mouth; a primary electro-acoustic driver mounted at or adjacent to the throat of the horn and directed generally along the horn; and at least one secondary electro-acoustic driver mounted party-way along the horn, spaced from the throat, and directed generally across the horn.
There may be a signal conditioning means for conditioning input signals to at least one said secondary driver to control the polar response of the horn loudspeaker.
In accordance with a second aspect of the present invention, there is provided a horn loudspeaker system, comprising: a horn having a throat and a mouth; a primary electro-acoustic driver mounted at or adjacent to the throat of the horn and directed generally along the horn; at least one secondary electro-acoustic driver in a side surface of the horn and directed generally across the horn; and means for processing input signals to at least one said secondary driver to control the polar response of the horn loudspeaker.
The signal processing means may process an input signal for the primary driver to produce a processed signal for the or each secondary driver.
The signal processing means may select at least one frequency component (frequency band) of the input signal for processing.
The signal processing means may be chosen or programmed (e.g. if it is a digital filter or other digital signal processor) so as to optimise some aspect of the polar response of the horn loudspeaker, for example to increase directivity, to flatten the polar response within a specified included radiation angle (for example approximating an ideal n0 x n0 perfect radiator), or to increase omnidirectionality. Means are preferably provided for adjusting the filtering or other processing characteristic of the signal processor, for example so that the polar response of the horn loudspeaker can be selected at the flick of a switch or twist of a knob. The system may further include: means for amplifying the input signal for supply to the primary driver; and means for amplifying the processed signal(s) for supply to the secondary driver(s). The signal processing can then be done at line level.
In a preferred form of the invention, the signal processing means comprises frequency selective networks (filters), implemented using either conventional (analog) or discrete time (digital) technologies. Each filter response is designed to provide an appropriate ratio between the electrical signal to the primary driver and the electrical signal to the secondary driver(s). This ratio ultimately determines the acoustic impedance at the surface of the primary and secondary driver(s) thus influencing the radiation load presented to the primary driver and the overall directivity of the horn loudspeaker.
There may be a range of user-selectable filter settings to give a single horn a range of directivity patterns.
The response of each filter may be designated by setting the filter parameters by i) manual adjustment, or ii) explicit optimisation (eg. Wiener Optimal Filtering) or iii) automatic numerical optimisation routines (e.g. Genetic Algorithms).
Preferably at least two such secondary drivers are provided. In this case, the secondary drivers are preferably arranged as one or more pairs, at least one of the drivers of each pair being arranged generally symmetrically with respect to the horn axis and having their electrical inputs connected in phase with each other. Thus the secondary drivers do not affect the acoustic axis of the horn loudspeaker. One such pair of secondary drivers may be provided, but preferably at least two such pairs are provided. In this case, the secondary drivers of a first of the pairs are preferably directed generally in a first plane generally across the axis of the horn; and the secondary drivers of a second of the pairs are preferably directed generally in a second plane, generally at right angles to the first plane, generally across the axis of the horn. Thus, for example, the polar response can be altered in both azimuth and elevation. Also, the signal processing means is preferably arranged to produce a first such processed signal for one of the pairs of secondary drivers and a second such processed signal for another of the pairs of secondary drivers. Accordingly, the azimuthal and elevational responses can be altered in different ways.
Preferably, the secondary driver, or at least one of the secondary drivers, is disposed nearer the mouth than the throat of the horn, which preferably has an exponential or hyperbolic taper.
Preferably, the primary driver or each of the secondary drivers is mounted in the wall of the horn and is directed generally at right angles to the portion of the wall in which it is mounted.